Build: Build stub functions of system calls when required

Change-Id: I286bec82f63d97175d12e486330f62bea5a0c449
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
diff --git a/tests_reg/CMakeLists.txt b/tests_reg/CMakeLists.txt
index 091a5ec..330566a 100644
--- a/tests_reg/CMakeLists.txt
+++ b/tests_reg/CMakeLists.txt
@@ -69,6 +69,7 @@
 target_sources(tfm_ns
     PRIVATE
         test_app.c
+        $<$<BOOL:${CONFIG_GNU_SYSCALL_STUB_ENABLED}>:../app_broker/syscalls_stub.c>
 )
 
 target_link_libraries(tfm_ns